ParenthoodPregnancyMo/MoTwins:Definition,Risks,andMoreMedicallyreviewedbyCarolynKay,M.D.—WrittenbyCatherineCrideronOctober14,2020WhattheyareDiagnosisRisksTreatmentsTakeawayShareonPinterestKikeArnaiz/StocksyUnitedInmostcases,thebiggestdistinctionpeoplemakebetweentwinsiswhetherthey’reidenticalorfraternal(andeventhatseparationcanbeconfusingforsome).Ifupuntilthispointtwinshavebeenageneticmysteryforyou,pleaseallowustointroduceyoutosomeofthevarioustypesoftwins.Oneofthemostrareandcomplicatedtypesaresometimescalledmo/motwins.Confused?Intrigued?Keepreadingtofindoutmore!What are mo/mo twins?Mo/moisashortenedformofmonochorionicmonoamniotic.Mo/motwinsarethosewhoshareboththechorionicandamnioticsacs.Ortoputitanotherway,sincemonomeans“one,”amo/mopregnancyisonewherethereisoneplacentaandoneamnioticsacforbothbabies.Confusedabouthowthisworks?Allowustobackupabit…Mostpeoplehaveheardmentionofidenticalandfraternaltwinsbefore,right?Identicaltwins(alsocalledmonozygotictwins)comefromthesamefertilizedegg.Theyarecreatedwhenoneeggandonespermmeetasexpected.Shortlyafterfertilization,thesingleeggsplitsintwo.Thesetwinsarereferredtoasidentical,becausetheyshareallthesamechromosomes.Fraternaltwins(alsocalleddizygotictwins)ontheotherhandcomefromtwodifferenteggsthatarereleasedclosetogetherandfertilizedbytwodifferentsperm.Theyshareonly50percentoftheirchromosomesasothersiblingsdo,andcharacteristicslikehairandeyecolororgendercanbethesameordifferent.Gotallthatandwonderingwhatthathastodowithmo/motwins?Well,believeitornot,twinscanshareafewotherthingsbesideschromosomes!Whenitcomestoidentifyingtypesoftwins,classificationdependsonwhetherthefetusessharedorhadseparatechorionicandamnioticsacs.Neverheardofachorionoramnioticsac?Don’tworry,you’renotalone,sohere’saquickrecapandafewnotesonwhytheyareimportant.Whiledevelopinginthewomb,thechorionistheoutermostfetalmembrane,theouterlayerofwhatisknownastheamnioticsac.Thechorionworkstoconnecttheamnion,amnioticfluid,andthefetustotheplacenta.Theamnionistheinnermostfetalmembrane